The Applicability of NMR Spectroscopy in the Determination of Structures Containing a Fragment of Spiro[1-Azabicyclo[3.3.0]octane]

Structures of two new spiro-adducts synthesized on the basis of [3 + 2]-cycloaddition reaction of stable azomethine ylide with styrene and 2,3-dihydrofuran, respectively, were defined by NMR methods. Complete signal assignments in ¹H spectra of both compounds were obtained on base analysis of COSY and NOESY experiments by determination of scalar networks in the first case and spatial interactions in the second one, respectively. The identification of aromatic proton signals of indan-1.3-dione fragment was made on base registration of long-range NOEs between them and aliphatic protons which are located at the distances up to 6 angstroms. These proton assignments were helpful for the identification of carbon signals in ¹³C NMR spectra of compounds under study which were performed by combination of HSQC and HMBC data. Thus, the results of the consistent use of homonuclear (COSY, NOESY) and heteronuclear (HSQC, HMBC) allowed us to make a complete and unambiguous NMR description of two new spiro-adducts and prove their belonging to one of the four synthetically possible regioisomers: (2'SR,7a'RS)-2'-phenyl-1',2',5',6',7',7a'-hexahydrospiro[indene-2,3'-pyrrolizine]-1,3-dione) and (3aSR,3bSR,8aSR)-2,3,3a,3b,4,5,6,8a-octahydrospiro[furo[3,2-a]pyrrolizine-8,2'-indene]-1',3'-dione). The possibility of the existence of the second regioisomer in solution in the form of a fast (in the NMR time-scale) conformational exchange associated with pseudo-rotation around the CH₂–CH₂ bond in the ethane fragment of the oxygen-containing five-membered ring was detected.